Police and military forces throughout the world are confronted with the problem of detecting explosives. Optimally, detection of explosives could occur at a distance by sampling volatiles in the air for explosives.
Several types of chemo-electronic sensors are currently used to detect explosives. Fluorescent polymers that react to volatile chemicals, such as nitrogen-containing compounds from explosives, can detect trinitrotoluene (TNT) in the femtogram range by measuring increases or decreases in fluorescence when the explosive interacts with the fluorescent polymer. Sensor arrays, comprising thousands of microsphere sensors, each responding to a particular chemical compound or class of compounds by fluorescing at different wavelengths, can detect TNT and 2,4-dinitrotoluene (2,4-DNT) in the low parts per billion range. An array of different polymeric thin film sensors, which responds to numerous chemical compounds to produce a pattern of electrical resistance changes analyzed by a neural network, can detect 2,4-DNT at a level of about 0.2 parts per billion. A chemiresistor vapor sensor, based on semiconducting films of alkanethiol-stabilized gold nanoclusters deposited on an interdigitated microelectrode array, can measure changes in electrical conductivity when exposed to vapors of dinitrobenzene, 2,4-DNT, and TNT at room temperature. A surface acoustic wave (SAW)-based gas chromatograph can detect 2,4-DNT at an estimated limit of 92 parts per trillion.
Greater sensitivity than that achieved by the explosive detectors outlined above would allow detection of explosives at a greater distance, explosives with a low vapor pressure, explosives packed in a container, for example, land mines or improvised explosive devices (IEDs), and the reduction of false positive detections. There exists a requirement for the sensitivity of explosive detectors to detect a cluster of molecules or even a single molecule. In the above explosive detectors, however, the inability to detect the interaction of single explosive molecule with a sensing molecule, that is, a single binding event, is due to their inability to provide an adequate method of amplification. Typically, electronic amplification is inadequate because of the poor signal-to-noise ratio.
Recent research (D. M. Rosenberg et al., “Chemical neutralization of nitroarene-based energetic mixtures: Optimization of the hypergolic reaction with amines based on a structure/reactivity study”, 230th American Chemical Society National Meeting, Washington, D.C., Aug. 28-Sep. 1, 2005), indicates that a nitroarene explosive molecule, for example, 2,4-DNT or TNT, reacts with a polyamine molecule, for example, diethylenetriamine (DETA), in an irreversible capture by means of covalent bond formation. As shown in FIG. 1, the reaction product formed is a zwitterionic Meisenheimer complex having a labile proton. Given the exothermic nature of the reaction between the nitroarene explosive and the polyamine, as shown in FIG. 2, the resulting increase in kinetic energy imparted to the region of proton exchange about the nitroarene point of attachment, may induce greater and/or more rapid separation of electrical charges.
As shown in FIG. 3, the effect of adsorbed vapors on the electrical properties of thin films is often studied with a microelectrode array. Although FIG. 3 illustrates a periodic interdigital microelectrode array, that is, an array where the distance between the microelectrodes is a constant, non-periodic microelectrode arrays are also used. Other geometries of microelectrode arrays, for example, concentric annular arrays, are also used to study the electrical properties of thin films.
The interdigitating microelectrodes, shown in FIG. 3, are formed on a semiconductor substrate by well known semiconductor technology methods. A thin film of the material under study is deposited over the microelectrode array and the semiconductor substrate. The underlying interdigital microelectrode array forms the equivalent of a parallel capacitor array, where electrical-field fringe effects penetrate the overlying thin film of the material under study. When vapors are adsorbed on the upper surface of the thin film, changes to the electrical properties, for example, resistance, conductance, and capacitance, of the thin film are measured across the electrodes. Time-varying currents and voltages can be applied to the electrodes, as is well known in the art, to facilitate the measurement of the electrical characteristics of the thin film.
Microelectrode arrays are small, inexpensive, rugged, and offer a well known method for studying changes in electrical characteristics of a thin film of a material under study, when vapors are adsorbed by the thin film.
There remains a need for an explosive detector that may be small, inexpensive and rugged, and that may detect a cluster of molecules or even a single molecule of an explosive.